The present invention relates, in general, to electronics, and more particularly, to methods of forming power supply systems and structures.
In the past, the electronics industry utilized various methods and structures to form power supplies that had multiple output voltages. In most cases, a transformer and with multiple secondary windings was used to provide the multiple output voltages. For example in applications such as a television, it was desirable to have a high voltage output that was used for the visual display portion and a lower voltage output that was used for control elements or control logic. It was also desirable to have a standby mode that reduced the voltage on the high voltage output maintained the voltage on the low voltage output so that the control logic operated. One example of such a power supply system was disclosed in U.S. Pat. No. 4,688,159 issued to Antonius Marinus on Aug. 18, 1987. One problem with these prior power supply systems was that control circuitry was required to control the semiconductor switches used for the different operating modes of the system. Additionally, it was difficult to have smooth transitions between the different modes.
Another configuration used a linear regulator connected to one of the high voltage winding. One problem with these prior power supply systems was that if the linear regulator failed it was possible to overheat or damage at least some of the power supply system. In order to reduce the damage, current limiting protection resistors were added to protect the power transistors. However, the current limiting protection resistors cause higher power dissipation in the standby operating mode. Additionally, the protection resistors also limited the minimum voltage to which the output could be reduced which also increased power dissipation. Another way to minimize the damage was to connect the linear regulator to a lower voltage winding, but this also increases the standby power dissipation.
Accordingly, it is desirable to have a power supply system that operates in a standby mode with reduced power dissipation, and that can reduce the value of the high voltage output.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-Channel devices, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention.